Kumano G, Nishida H
Department of Life Science, Tokyo Institute of Technology, Yokohama, Japan.
Dev Biol. 1998 Jun 15;198(2):245-52.
Alkaline phosphatase (AP) activity is expressed by endodermal cells of ascidian larvae. It was reported previously that the expression of AP activity is resistant to treatment with actinomycin D, a transcription inhibitor that inhibits the appearance of several other tissue-specific molecules and morphological markers of tissue formation in developing ascidians. The resistance of AP expression to actinomycin D treatment suggests that endodermal AP activity does not depend on zygotic transcription and that its appearance is mediated by the translational activation of maternal AP mRNA present in ascidian eggs. However, it was also shown that anucleate merogons do not develop AP activity. To directly examine whether maternal AP transcripts are present in the cytoplasm of eggs, we isolated a cDNA of an endoderm-specific AP in Halocynthia roretzi and examined the temporal and spatial expressions of this gene during embryogenesis using Northern blots and in situ hybridization. Maternal AP transcripts were detected in oocytes, cleaving-stage embryos, and in gastrulae, and endoderm-specific AP transcripts dramatically increased about 14 times from the neurula stage to the larval stage in endoderm precursor cells. These results suggest that the differentiation of endoderm is primarily correlated with the activation of zygotic transcription of the AP gene, presumably by egg cytoplasmic factors, similar to how muscle and epidermis are believed to develop.
碱性磷酸酶（AP）活性由海鞘幼虫的内胚层细胞表达。此前有报道称，AP活性的表达对放线菌素D处理具有抗性，放线菌素D是一种转录抑制剂，可抑制发育中的海鞘中其他几种组织特异性分子的出现以及组织形成的形态学标记。AP表达对放线菌素D处理的抗性表明，内胚层AP活性不依赖于合子转录，其出现是由海鞘卵中存在的母体AP mRNA的翻译激活介导的。然而，也有研究表明无核卵块不产生AP活性。为了直接检测母体AP转录本是否存在于卵细胞质中，我们分离了罗氏海鞘中内胚层特异性AP的cDNA，并使用Northern印迹和原位杂交技术检测了该基因在胚胎发育过程中的时空表达。在卵母细胞、分裂期胚胎和原肠胚中检测到母体AP转录本，并且在内胚层前体细胞中，内胚层特异性AP转录本从神经胚期到幼虫期急剧增加了约14倍。这些结果表明，内胚层的分化主要与AP基因合子转录的激活相关，推测是由卵细胞质因子介导的，类似于肌肉和表皮的发育方式。
